Author: Larry A. DeWerd, Sean Jollota, Karen Rex Pius Vincent 👨🔬
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison 🌍
Purpose: Absolute measurements, often used to establish primary standards, offer lower measurement uncertainty. For 192Ir high-dose-rate (HDR) brachytherapy, the interim primary standard for air-kerma strength relies on a non-absolute measurement using the seven-distance technique. This method requires an ionization chamber with a calibration coefficient interpolated between two NIST-traceable quantities (M250 x-ray beam and 137Cs) to approximate the effective gamma emission energy of 192Ir. This work uses an Exradin A3 known-volume ionization chamber to perform absolute measurements of air-kerma strength for a 192Ir HDR source and compares these results with those obtained using the interim standard.
Methods: The sensitive volume of an Exradin A3 ionization chamber was determined using micro-CT imaging, with corrections applied for electric field inhomogeneity and magnification effects. EGSnrc Monte Carlo simulations were performed to calculate the radiative fraction, mass energy-absorption coefficient ratio, and stopping power ratio. Experimental corrections accounted for temperature and pressure, ion recombination, and electrometer calibration, while axial and radial nonuniformity corrections were inferred from published literature. Using these parameters, the known-volume chamber directly measured the air-kerma rate, and the seven-distance method was applied to calculate the in-vacuo air-kerma strength. The results were compared to those obtained using the interpolated calibration coefficient and corrected charge reading as specified by the interim standard.
Results: The air-kerma strength measured with the known-volume ionization chamber was 2.889 × 10-2 Gy×m2/hr compared to 2.892 × 10-2 Gy×m2/hr from the interpolated calibration coefficient. The relative standard uncertainty (k= 1) was 0.71% and 1.06%, respectively. The resulting percentage difference between these measurement techniques was 0.081%.
Conclusion: The known-volume ionization chamber demonstrates that the interpolated calibration method has been an excellent approximation to determine the air-kerma strength of a 192Ir HDR brachytherapy source. The known volume chamber has a higher metrological quality compared to the interpolated calibration method.